Or operator for custom taxonomies, need help…

I have a page that allows the user to select two custom taxonomies to search custom post types by (State and Category).

But when the user only selects only one of the two taxonomies and submits a search: the results are always empty even though there ARE custom posts associated with them. I’ve noticed that when this happens, the URL will have state-and-city=-1 or categories=-1 for the taxonomy that was NOT selected. But if I remove the -1 and hit return, then it WILL show results properly.

So I tried to look for a parameter that lets me define the value blank instead of the -1 but there doesn’t seem to be one that exists. Can anyone help? Thanks!